Synchronous motor with polarized rotor



Oct. 5 1954 c. F. CLIFFORD ETAL 2,591,112

SYNCHRONOUS MOTOR WITH POLARIZED ROTOR /M/E/VTOES CEc/,c F, CL /FFORD 554/5 6./7- p19 WYE' Oct 5 1954 c. F. CLIFFORD ETAL 2,691,112

sYNcHRoNoUs MOTOR WITH POLARIZED RoToR Filed March 10, 1952 2 sheets-sheet 2 Patented Oct. 5, 1954 SXNCHRONOU'S; MOTQR VYITHy POLARIZED KOTOR Cecil Frank Clifford and Leslie George Mountjoy Payne, Bath, Somerset, England Applieation.March 1.0, 1952 S erial No.. 27.57 62.

Claims priority, applica-tion- Great Britain March 14, 1951 4 Claims.Y 1

This invention relates ta small synchronous electric motors of the wel-l knownv type, common ly used: for driving synchronous: clocks,y in which the rotor is permanentlyI magnetisedf and is: designed to start spontaneously when the current is switched on'.v

The object of the invention is to improve the starting properties of such motors.

In motors of the type' above referredr toas at present used, the rotor normally comes to rest with its poles directly opposite the stator poles, because' thel positions of the rotor in which the rotorv poles arel directly opposite the stator poles are the positions of mini-mum magnetioreluctance of the gapsl between the rotorand stator poles. rIhisr isthe most disadvantageous position for starting, and it is only the inherent instability of the equilibrium of the rotor; during theY halfcycles in which it is acted: upon by magnetic forces of repulsion,l that enablesv such motors Ito start .when the current is switched on.

According tothe present invention',v the rotor is constructed soA that in the positions of minimum magnetic reluctance: between the rotor and stator poles, the, rotor poles are so displaced relatively tof the stator poles as to'r be sub-ject to attractive and repulsive forces which exert an immediate starting torque on the rotor when the stator current is switched on. A positive and very reliable starting action is thus achieved.

The. poles may be arranged so that the magnetic reluctance of the gaps between the rotor and stator poles falls .to a minimum when. the rotor poles have their axesA opposite the gaps between the stator poles, but this is not essential because the desired effect is obtained so long as the position of minimum reluctance to which the rotor is naturally attracted when the current is switched ofi is angularly displaced from the position towards which the roto-r is attracted when the current is switched on.

The invention will be fully understood from the following more detailed descriptionof two examples of constructions embodying the invention, reference being made to the 'accompanying drawing-s in which:

Figure l is a sectional View of a motor taken on a plane at right-angles to the rotational axis of the motor;

Figure 2 is a section taken on a lplane containing the rotational axis;

Figure 3 is a diagram drawn to a larger scale showing the arrangement of the stator and rotor poles, and

Figure 4 is a View corresponding to Figure 3 Z showing a modied arrangement of the rotor poles.

The. motor shown in- Figures l and 2 hasA a stator of a well known type of construction con-- sisting; of a hollow annulary shell I` composed of two halves which encloses a coaxial magnetising colli t.. The; two; halves of the shell are separated byr an annular gap extend-ing round the inner Wall of the shell, and the stator p-oles are formed Joy two sets of teeth 3i. and i formed respectively on the twoy halves' of the shell and projecting towards; one another across the gap just referred to.v The teeth v3 of one set lie opposite the spaces, between the teeth Il of the other set so that teeth 3 belonging to one set alternate with teeth 4: belonging to the other set round the inner Iwall of the stator. When current is supplied to.v the coil 2, the two ysets of teeth are oppositely pol-arised and poles of opposite polarity thu-s occur alternately round the stator.

The motor has. a rotor 5 consisting of a bar or sheet metal plate xed to a rotatable spindle 6 supported in suitable bearings 'l coaxial with the shell t. The rotor 5 is permanently magnetised so thaty its two ends are of opposite polarity; It is formed at one end? with teeth 8 and at the other end with teeth 9 forming polar projections for `co/-operatingv with the stator poles, the teeth at each end (being of like polarity) 'being spaced. apart by a distance equal to twice the pitch of the stator teeth (i. e. equal to the pitch oi either of the two separate sets of stator teeth 34 and 4),. The extremities of the teeth 3 and il may be bent over as shown in Figure 2 to increase the effective area of their polar surfaces.

In the construction shown in Figures l. 2 and 3, the rotor 5 is symmetrical in the sensel that each tooth 8 att one end of the rotor corresponds to a tooth 9 of opposite magnetic polarity situated in `a diametrically opposite position. This requires that stator poles of opposite magnetic polarity should occur in diametrically opposite positions round the stator. The stator will thus require to have an odd number of teeth in each of the two sets 3 and 4. For example there may be fifteen teeth 3L' and fteen teeth 4, making thirty stator poles in all.

As shown in Figures 1, 2Y and 3, each of the teeth 3, and il forming the rotor poles is made wider than the teeth 3 and l forming the stator poles and is bifurcated or divided by forming notches t0 in the rotor. The. two polar surfaces 8a and 8b or 8a and `9h (see Figure 3) into which each rotor pole is thus divided are separated Sufciently to ensure that the magnetic reluctance of the gaps between the rotor and stator poles is at a maximum when the axes of the rotor poles (marked :c in Figure 3) are in alignment with the stator poles, and falls to a minimum ywhen the axes :c are in line with the spaces Ibetween the stator poles. This can be readily understood by inspection of Figure 3 of the drawings, in which the rotor is shown in full lines in a position of minimum magnetic reluctance between the rotor and stato-r poles and is shown for comparison in broken lines in a position in which the axes r of the rotor poles are aligned with the stator poles.

Owing to its being constructed as described above, the rotor 5 naturally asumes a position such as that shown in full lines in Figure 3 when the current is switched off. Consequently, when the current is switched on and the stator teeth 3 and II are thereby oppositely polarised, each tooth l and 8 is subjected to attractive and repulsive magnetic forces which exert an immediate starting torque on the rotor when the current is switched on.

Instead of being bifurcated as described above and as shown in Figure 3, each tooth 8 or 9 may be so magnetised as to form two strongly polarised surfaces of like polarity corresponding to the polar surfaces 8a and 8b (or 9a and Sb) separated by an unpolarised or relatively weakly polarised region located at the axis of the pole.

The width of each rotor pole 8 and 9 preferably made equal to the pitch of the stator poles, the notch I t or the unmagnetised interval between two strongly polarised surfaces being approximately equal to the width of the stator poles 2 and 3.

Figure 4 of the drawings shows an arrangement of rotor poles in which the desired result is obtained by displacing some of the rotor poles from their normal pitch positions relatively to the remaining rotor poles instead of by bifurcating the rotor poles.

As shown in Figure 4, the stator poles 3 and 4 are like those described with reference to Figures i, 2 and 3 except that, for convenience, the number of poles in each set 2 and 3 is an even number in Figure 4, whereas an odd number was used in Figures l to 3.

The rotor 5 of Figure 4 is permanently magnetised so that its two ends are of opposite polarity, and it is formed with teeth or projections II at one end and with teeth or projections I2 at the other end, the teeth I I and I2 forming rotor poles.

The teeth II, which are of like polarity, are spaced apart by a distance equal to twice the pitch of the stator teeth (i. e. equal to the pitch of either of the two sets of teeth 2 and 3 taken separately). The teeth I2, which are also of like polarity, are also spaced apart from one another by a distance equal to twice the pitch of the stator teeth, but they are displaced from their normal pitch positions relatively to the teeth I I.

The normal pitch positions of the teeth I2 relatively to the teeth II above referred to are the positions that the teeth I2 would assume in a motor of normal design, i. e. are so chosen that the angular positions of the teeth I2 with respect to teeth 4 of one polarity correspond with the angular positions of the teeth II with respect to the teeth 3 of opposite polarity (having regard to the fact that the teeth II and I2 are of opposite polarity). In order to make this clear, a possible normal pitch position of the teeth I2 on centre lines y corresponding to the centre lines :n of the teeth II has been shown in broken lines at A in Figure 4. It will be seen from the figure, that the teeth I2 are displaced from this normal pitch position by a distance equal to about half the pitch of the stator teeth.

Owing to the relative displacement of the rotor poles Il and I 2 described above, the magnetic forces exerted during a single half cycle of the operating current tend to move the rotor towards a position such as that shown in broken lines at B in Figure 4. The position shown at B in Figure 4 corresponds to the position of the rotor shown in broken lines in Figure 3. Both figures show the rotor at B approximately in one of the positions towards which the rotor is attracted during one half-cycle of the operating current.

To obtain the result aimed at by the present invention, these positions must be different from the positions of minimum magnetic reluctance between the rotor and stator poles. In the construction shown in Figures 1 to 3, this is achieved by the notches IIJ. To achieve the same result in the construction shown in Figure 4, the rotor teeth IE and II are cut away on one side as shown at I3 to reduce the magnetic reluctance between the rotor and stator poles in positions of the rotor such as that shown at B.

When the stator current is switched off, therefore, the rotor is naturally attracted to a position of minimum magnetic reluctance such as that shown in full lines in Figure 4 corresponding to the rotor position shown in full lines in Figure 3. In this position, the teeth II and I2 are subject to magnetic forces which exert an immediate starting torque when the stator current is switched on.

In both forms of the invention shown in Figures 3 and 4, the number of teeth 2 and 3 or II and I2 may be varied from a minimum of two in all to the maximum number that can be accommodated according to the number of stator teeth.

It is to be noted that the notches I0 shown in Figure 3 and the cut-away portions I3 shown in Figure 4 both come opposite stator poles 3 or 4 in positions of rotation (such as those shown in broken lines) towards which the rotor is attracted during alternate half-cycles of the operating current. In both of the forms of the invention shown in Figures 3 and 4 therefore, the cut-away portions are eiective to ensure that the magnetic reluctance of the air gaps between the rotor and the stator poles rises to a maximum in the successive positions towards which the rotor is attracted as aforesaid.

What we claim is:

l. A synchronous electric motor comprising a polarised rotor with north and south poles and a stator formed by an alternating magnet having two sets of teeth forming alternate stator poles evenly spaced round said stator and eiective for attracting said rotor towards successive positions of rotation during successive half-cycles of operating current, said rotor and stator teeth having the same pole pitch unchanged throughout the circumference, wherein said rotor has teeth forming rotor poles having weakened or cut away portions to ensure a higher reluctance when the axis of the rotor poles lie radially opposite the stator poles than when the axis of the rotor poles lie between the stator poles, thus ensuring that the rotor comes to rest in the latter position where it will receive the maximum possible starting couple when the stator is energised with alternating current.

2. A synchronous electric motor comprising a polarized rotor and a stator formed by an alterhating-current magnet having two sets of teeth forming alternate stator poles evenly spaced round said stator and effective for attracting said rotor towards successive positions of rotation during successive half-cycles of operating current, wherein said rotor has teeth forming rotor poles spaced apart by a distance equal to twice the pitch of the stator teeth, said rotor teeth being made wider than said stator teeth and each having a weakened or cut-away middle portion that cornes opposite the stator poles in the successive positions of rotation towards which the rotor is attracted as aforesaid.

3. A synchronous electric motor according to claim 2 where said rotor teeth are bifurcated.

Number Name Date 1,913,506 Nachumsohn June 13, 1933 1,933,590 Holtz et al Nov. 7, 1933 1,966,897 Lofgren July 17, 1934 1,976,880 Graseby Oct. 16, 1934 FOREIGN PATENTS Number Country Date 28,539 Great Britain of 1902 388,739 Great Britain Feb. 28, 1933 451,447 Great Britain Aug. 6, 1936 781,076 France Feb. 18, 1935 

